FIG. 1 shows a prior art active signal cancellation loop 8 added to a filter-based duplexer 12 to improve isolation between a transmitter TX and a receiver RX. More precisely, the cancellation loop 8 comprises a first coupler 10, a unit 14 comprising a variable attenuator and a phase shifter, and a second coupler 16.
The first coupler 10 samples a signal generated by the transmitter TX. The variable attenuator controls the amplitude of the sampled signal generated to provide a signal which is then shifted in phase using the phase shifter. The resulting signal shifted in phase is then injected at the receiver using the second coupler 16. The skilled addressee will appreciate that as the phase is shifted by 180 degrees, the second coupler 16 acts as a subtraction unit which removes a signal leaking from the transmitter TX to the receiver RX.
The skilled addressee will appreciate that in order to cancel the signal leaking from the transmitter TX to the receiver RX, it is necessary that the resulting signal shifted in phase and injected by the second coupler 16 has an amplitude equal to the amplitude of the signal leaking and a phase shifted by 180 degrees. Unfortunately, the skilled addressee will appreciate that such condition is only achieved for a given frequency with the apparatus disclosed in FIG. 1. This topology is therefore inherently narrow band and is highly sensitive to the phase response of the duplexer path. The phase response is particularly problematic in the case of SAW filters. Therefore, if the topology of FIG. 1 is used, the transmitter TX receiver RX isolation is expected to be improved over a very narrow band. Such improvement may be significant but would have to be adjusted dynamically through the control of the variable attenuator and the phase shifter by the cell phone to place the cancellation null at the user's exact frequency in the case where the transmitter and the receiver are embedded in a cell phone. Such type of improvement has already been demonstrated with other filters and is reproduced here using a commercial duplexer (part number 856356 from SawTek). It has also been contemplated that if the antenna to which the transmitter and the receiver are connected is not well adapted, the cancellation loop 8 has only a limited effect on the signal leaking from the transmitter to the receiver.
The results are shown in FIGS. 9a, 9b, 10a and 10b. As illustrated, FIGS. 9a, 9b, 10a and 10b show that a high level of isolation may be achieved over a narrow bandwidth, which is typically around 1-2 MHz. It will be appreciated that this cancellation may be tuned to the desired frequency by adjusting mainly the phase shifter. While this topology is simple, the access to active tuning and the adaptive control of the tuning require the interface to the digital processing power of the receiver.
Referring to FIG. 2, there is shown another embodiment of a prior art active signal cancellation loop 17 added to a circulator-based duplexer 24 to improve isolation between a transmitter TX and a receiver RX. More precisely, the cancellation loop 17 comprises a first coupler 18, a unit 17 comprising a variable attenuator and a phase shifter, and a second coupler 22.
The first coupler 18 samples a signal generated by the transmitter TX. The variable attenuator controls the amplitude of the sampled signal generated to provide a signal which is shifted in phase using the phase shifter. The resulting signal shifted in phase is injected at the receiver using the second coupler 22. The skilled addressee will appreciate that as the phase is shifted by 180 degrees, the second coupler 22 acts as a subtraction unit which removes a signal leaking from the transmitter TX to the receiver RX. The skilled addressee will appreciate that the circulator-based duplexer 24 provides a less important transmitter receiver isolation which therefore cause a greater signal leaking. The cancellation loop 17 therefore brings an improvement while still suffering from the same drawbacks outlined in the embodiment disclosed in FIG. 1.
There is a need for a method and apparatus that will overcome at least one of the above-identified drawbacks.
Features of the invention will be apparent from review of the disclosure, drawings and description of the invention below.